1. Field of the Invention
The present invention relates to a display device, and more particularly, to an electroluminescent device and a method for manufacturing the same.
2. Discussion of the Related Art
Ultra thin sized flat panel displays having a display screen with a thickness of several centimeters, especially liquid crystal display (LCD) devices, are widely used for monitors in notebook computers, spacecraft, aircraft, etc.
LCD panels are in general non-luminous and require a backlight at the rear of the liquid crystal panel as a light source. The conventional backlight is not satisfactory because of its large weight, power consumption, and thickness. In this respect, it is desirable to replace the conventional backlight with a thinner, lighter, less-power consuming alternative. Currently, thin and light electroluminescent devices are under research and development.
Electroluminescent devices can be divided into two types: a light-emitting diode (LED) and an electroluminescent diode (ELD), depending on the operational principles. The light emission of LEDs is based on a radiant transition due to electron-hole recombination near a P-N junction. Recently, a rapid development of an LED based on an organic material is in progress.
On the other hand, the light emission of ELDs is based on luminescence that takes place when high energy electrons generated in a light-emitting layer excite a phosphor upon impact. Electron within the light-emitting layer acquire energy from a high electric field and turn into hot electrons. The hot electrons then excite an activator to generate light.
ELDs are manufactured by thick-film printing of a mixture of resin and light-emitting powder or by thin film printing. ELDs are also divided into two types: the AC type and the DC type, depending on the driving modes.
An electroluminescent device of the related art will be described with reference to FIG. 1. FIG. 1 is a schematic perspective view of a related art electroluminescent device. As shown in FIG. 1, the related art electroluminescent device includes a substrate 11 and a transparent electrode layer 13 on the substrate 11. The transparent electrode layer 13 is formed in a predetermined pattern, such as in a stripe pattern. The transparent electrode 13 is formed of indium tin oxide (ITO), for example. A lower insulating layer 15 of SiOx, SiNx, or BaTiO3 is formed on the transparent electrode layer 13, and a light-emitting layer 17 of a ZnS based light-emitting material is formed on the lower insulating layer 15. The related art device further includes an upper insulating layer 19 made of SiOx, SiNx, or Al2O3 on the light-emitting layer 17. It further includes a metal electrode layer 21 made of a metal, such as Al, on the upper insulating layer 19, and a surface passivation layer 23 on the metal electrode layer 21.
In this related art electroluminescent device, when all AC voltage is applied between the transparent electrode layer 13 and the metal electrode layer 21, a high electric field in the order of 106 V/cm is built within the light-emitting layer 17. Electrons generated in the interface between the upper insulating layer 19 and the light-emitting layer 17 tunnel into the light-emitting layer 17.
The tunneling electrons are accelerated by the high electric field in the light-emitting layer 17. The accelerated electrons collide with activators (Cu and/or Mn) within the light-emitting layer 17 to excite electrons in the ground state to excited states. When electrons at a higher energy level transit to the vacant sites in a lower energy level state created by the excitation—e.g., when the excited electrons transit to the ground state (or to other lower energy level states), light having a wavelength corresponding to the energy difference is emitted. The color of the emitted light thus depends on the energy difference.
A method for manufacturing the related art electroluminescent device will now be described in, more detail. The transparent electrode layer 13 is formed on the glass substrate 11. Specifically, a thin ITO film having a high conductivity and a good transparent physical characteristic is deposited on the substrate 11. The thin ITO film is then patterned by photolithography into a stripe shape to form transparent electrodes, which are collectively referred to as “transparent electrode layer 13.”
A BaTiO3 based lower insulating layer 15 is formed on the transparent electrode layer 13 by RF reactive sputtering. The light-emitting layer 17 is then formed on the lower insulating layer 15. The light-emitting layer 17 may be formed via electron-beam deposition by cold pressing a powder of a Cu or Mn doped ZnS material and by generating small grains. Alternatively, the light-emitting layer 17 may be formed by sputtering using a target.
The upper insulating layer 19 of SiOx, SiNx, or Al2O3 is formed on the light-emitting layer 17 by sputtering or chemical vapor deposition (CVD). The metal electrode layer 21 is formed on the upper insulating layer 19. Specifically, a thin Al or Ag film is formed on the upper insulating layer 19 by thermal deposition and is patterned into stripe-shaped metal electrodes that extend perpendicularly to the transparent electrodes of the transparent electrode layer 13 underneath. Finally, the surface passivation layer 23 is formed on the metal electrode layer 21. This completes the manufacture of the related art electroluminescent device.
However, the related art electroluminescent device have several drawbacks. As briefly explained above, because a thin film transistor (TFT) liquid crystal display (LCD) panel. (TFT-LCD panel) for notebook computers and monitors has no self-luminous function, a light-emitting device such as a backlight is required. Since the conventional backlight is constructed by combining a light-guiding plate, a light-diffusion plate, and a prism with a cold cathode fluorescent lamp, the manufacturing cost is high, and the manufacturing process is undesirably complicated. Moreover, the large thickness of the backlights increases the thickness of the resultant monitor devices, which is undesirable, To substitute for such a conventional backlight, the related art electroluminescent device has been proposed. Although the manufacturing cost and thickness of the related art electroluminescent device have been somewhat reduced recently, it is still expensive. Moreover, the related art electroluminescent device still has an insufficient luminance to be used as a light source for LCDs.